This invention relates generally to semiconductor diode lasers and, more particularly, to one-dimensional arrays of semiconductor diode lasers fabricated as single structures. Single-element diode lasers are limited in power to outputs of the order of 30 milliwatts (mW), but arrays of diode lasers can be designed to provide output powers of hundreds of milliwatts. Such high power outputs are useful in optical communications systems, laser printers and other applications.
A survey of the state of the art of phase-locked laser arrays can be found in a paper entitled "Phase-Locked Arrays of Semiconductor Diode Lasers," by Dan Botez and Donald Ackley, IEEE Circuits and Devices Magazine, Vol 2, No. 1, pp. 8-17, Jan. 1986.
By way of general background, a semiconductor diode laser is a multilayered structure composed of different types of semiconductor materials, chemically doped with impurities to give them either an excess of electrons (n type) or an excess of electron vacancies or holes (p type). The basic structure of the semiconductor laser is that of a diode, having an n type layer, a p type layer, and an undoped active layer sandwiched between them. When the diode is forward-biased in normal operation, electrons and holes recombine in the region of the active layer, and light is emitted. The layers on each side of the active layer usually have a lower index of refraction than the active layer, and function as cladding layers in a dielectric waveguide that confines the light in a direction perpendicular to the layers. Various techniques are usually employed to confine the light in a lateral direction as well, and crystal facets are located at opposite ends of the structure, to provide for repeated reflections of the light back and forth in a longitudinal direction in the structure. If the diode current is above a threshold value, lasing takes place and light is emitted from one of the facets, in a direction generally perpendicular to the emitting facet.
Various approaches have been used to confine the light in a lateral sense within a semiconductor laser, i.e. perpendicular to the direction of the emitted light and within the plane of the active layer. Because of a requirement for a diffraction-limited beam, most research in the area has been directed to index-guided lasers. In these, various geometries are employed to introduce dielectric waveguide structures for confining the laser light in a lateral sense, i.e. perpendicular to the direction of light emission and generally in the same plane as the active layer. Most semiconductor structures employed for lateral index guiding in laser arrays are known as positive-index guides, i.e. the refractive index is highest in regions aligned with the laser elements and falls to a lower value in regions between elements, thereby effectively trapping light within the laser elements.
In general, an array of laser emitters can oscillate in one or more of multiple possible configurations, known as array modes. In what is usually considered to be the most desirable array mode, all of the emitters oscillate in phase. This is known as the fundamental or 0.degree.-phase-shift array mode, and it produces a far-field pattern in which most of the energy is concentrated in a single lobe, the width of which is limited, ideally, only by the diffraction of light. When adjacent laser emitters are 180.degree. out of phase, the array operates in the 180.degree.-phase-shift array mode, and produces two relatively widely spaced lobes in the far-field distribution pattern. Multiple additional modes exist between these two extremes, depending on the phase alignment of the separate emitters, and in general there are N possible array modes for an N-element array. For a ten-element array, the 0.degree.-phase-shift array mode is known as mode L=1, and the 180.degree.-phase-shift array mode is known as L=10. Many laser arrays operate in two or three array modes simultaneously and produce one or more beams that are typically two or three times wider than the diffraction limit.
One way to increase the power output of a laser array is to operate it at high current drive levels well above the lasing threshold. However, when evanescently-coupled devices that operate in the fundamental array mode at threshold level are driven in excess of 50% above threshold, their beams broaden, as a result of an effect known as gain spatial hole burning, and stable array-mode operation simply cannot be achieved. By providing for strong optical-mode confinement, using positive-index guiding for the elements, gain spatial hole burning is effectively suppressed, but so is the evanescent coupling between elements. One solution to this difficulty is to provide interelement coupling via Y-shaped branches. However, Y-branch coupling is relatively weak and results in emitted beams that are as much as four times larger than the diffraction limit.
Another solution was described, using wide-waveguide interferometric arrays, in application Ser. No. 07/233,390, filed August, 1988 by Dan Botez et al., issued as U.S. Pat. No. 4,866,724 entitled "Wide-Waveguide Interferometric Array with Interelement Losses.". This structure was deliberately designed to operate at a higher order array mode (L=8 for a 10-element array), which is stable against gain spatial hole burning, but has four lobes in its farfield pattern. The presence of four lobes may adversely affect the efficiency of beam transformation to a single lobe using .pi. phase shifters.
U.S. Pat. No. 4,860,298 to Dan Botez et al., entitled "Phase-Locked Array of Semiconductor Lasers Using Closely Spaced Antiguides," discloses another solution to the problem of gain spatial hole burning. In particular, lateral antiguiding provides both strong mode confinement and strong interelement coupling. The device operates at higher powers without being affected by gain spatial hole burning and with a desirable far-field distribution pattern. If powers in excess of 300 mW and up to 1 watt are desired for a particular application, this lateral antiguiding approach probably provides the best solution. However, the complexity of fabrication of the antiguiding structure results in a relatively costly device. Therefore, power outputs in the 200-300 mW range there is clearly a need for a simpler semiconductor laser array structure that will produce a practically diffraction-limited beam. The present invention fulfills this need.